Consumer electronic products such as televisions, digital cameras, cellular telephones, media content players, etc., are designed, manufactured and marketed for the purpose of satisfying the substantial consumer demand for basic communications and entertainment services. Data storage components play an important role in the operation of such devices. Data storage devices can include RAM, ROM, flash memory devices, etc.
Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed. Flash memory is primarily used in memory cards and USB flash drives for general storage and transfer of data between computers and other digital products. Flash memory is a specific type of EEPROM (Electrically Erasable Programmable Read-Only Memory) that is erased and programmed in large blocks. Example applications include data storage for PDAs (personal digital assistants), laptop computers, digital audio players, digital cameras and mobile phones. Other applications include game consoles, where flash memory can be used instead of other types of EEPROMs or battery-powered SRAM for game save data.
Flash memory is typically made up of an array of floating gate transistors that form respective memory cells of the array. One or more bits of data can be stored as charge by each of the memory cells. SONOS type flash memory cells include a silicon substrate that includes silicon oxide formed over portions of the silicon substrate, a layer of silicon oxide formed over the silicon portions, a layer of silicon nitride formed on the layer of silicon oxide, an upper layer of silicon oxide formed on the layer of silicon nitride, and a layer of electrically conductive material formed on the upper layer of silicon oxide. The lower silicon oxide layer, the silicon nitride layer and the upper silicon oxide layer (ONO stack) form a charge trapping dielectric structure.
FIG. 1 shows a cross sectional view of a portion of a memory cell array 100 that includes a SONOS structure. Memory cell array 100 includes a polysilicon wordline 101 that includes portions with sharp corners 103. The sharp cornered portions 103 are disposed in close proximity to isolation regions 107 that separate charge storage elements 109 in trenches which are associated with adjacent memory cells.
The memory cell configuration shown in FIG. 1 has significant shortcomings that can result in a degradation of performance of a memory cell array. Sharp polysilicon cornered portions 103 create a high electrical field that can cause the injection of electrons during erase operations and degrade core cells. Moreover, polysilicon material that is formed deep inside the trench is extremely difficult to remove during wordline definition. If such polysilicon is not removed it can result in an unintended and undesirable pathway for leakage current between adjacent memory cells that can cause the device to fail.